E85 (which is 85% ethanol) has a higher octane rating than pump 91 or 93 octane (AKI) gasoline. Turbocharged engines like the N63TU are often octane limited, so using a higher octane fuel allows the engine to run at higher boost (without reducing ignition timing) and make more power. E85 has a 30-35% lower stoichiometric ratio than gasoline, which means it requires 30-35% more fuel for the same amount of air for proper combustion. The extra fuel helps cool the combustion chamber which allows for more boost and power, but it also means the fueling system has to flow more fuel, which is often the limiting factor in how much ethanol can be used. For the N63TU with stock fueling components, E35 is the sweet spot between maximum octane and fuel system limitations (and its easy to remember the 2 to 1 ratio when filling up).